Elucidation of the molecular mechanisms governing cellular growth is central to our understanding of normal development and cancer. The goal of this proposal is to explore the regulation of the principal cell cycle checkpoint, the G1/S transition, in the context of the developing mouse lens. Our main focus will be on the regulation of a G1 cyclin-dependent kinase (cdk), cdk4, and its functional relationship to the retinoblastoma (Rb) protein. The cdk4 complex has captured center stage in the cell cycle field because of its documented importance in the proliferative response of cells to growth factors and of the involvement of its associated proteins in cancer pathogenesis. Activity of the cdk4 complex is regulated in large part through direct interactions with positive (cyclin D) and negative (inhibitors of cdk4, e.g. p15/INK4 and p16INK4) modulatory proteins. Owing to a foundation laid over the past 6 years, my laboratory is now uniquely positioned to address in a comprehensive manner the function of cdk4 and its associated regulatory proteins in normal mammalian development. This opportunity arises from (i) establishment of the lens as an ideal system for examining growth control in vivo (EMBO J, 1995) and demonstration that Rb, the substrate of cdk4, plays a central and essential role in lens cell growth control (Nature, 1994), (ii) characterization of the functional interrelationships among p16INK4, cdk4, Myc and Rb proteins in several cell culture systems (Science, 1995), (iii) generation of 4 different transgenic mouse lines (various cyclin D and cdk4 constructs) and a germline null mutation of the pl6INK4 gene, (iv) availability of cancer-prone transgenic mouse models that we and others have generated, and most importantly, (v) extensive experience in employing the mouse as an experimental system for the analysis of gene function. Characterization of this full complement of gain-of function and loss-of-function mouse models will allow us to understand the function of cyclin D, cdk4, and p15INK4, p16INK4 genes in the regulation of the G1/S transition in normal tissues in vivo and in the genesis of cancer. With respect to the cancer studies, we will examine and compare the onset and distribution of tumors arising in each mouse model separately, in combination with each other, or in conjunction with cancer- prone mice (such as p53-/- or some of our Myc transgenics). Finally, the connection of p16INK4 to familial melanoma will prompt an assessment of whether UV exposure of null p16INK mice accelerates the development of skin cancers.